Tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are important monomers in the preparation of fluoropolymers. TFE is commercially prepared by pyrolyzing CHClF2 (CFC-22) (U.S. Pat. No. 2,551,573). Hexafluoropropylene is commercially prepared by pyrolyzing TFE (U.S. Pat. No. 2,758,138). Thus on a commercial basis, the TFE and HFP are prepared sequentially. HFP is disclosed in other patents as being preparable by pyrolysis of a wide variety of fluorocarbons, e.g. a mixture of TFE and saturated fluorocarbon or C4 to C10 fluoroolefin (U.S. Pat. No. 2,970,176), TFE/carbon dioxide mixture (U.S. Pat. No. 3,873,630), 2-chloro-1,1,1,3,3,3-hexafluoropropane (U.S. Pat. No. 3,397,248), TFE, perfluorocyclobutane, or mixtures thereof, all in the presence of excess superheated steam (U.S. Pat. No. 3,446,858), and chlorotetrafluoroethane and/or chlorohexafluoropropane or a mixture of chlorotetrafluoroethane with perfluorocyclobutane (EP 0 337 127 A1). TFE and HFP are reported to be preparable simultaneously, i.e. co-synthesized, by pyrolyzing                a) chlorodifluoromethane to a mixture of TFE and HFP (U.S. Pat. No. 3,306,940),        b) a mixture of chlorodifluoromethane and 1,1,1,2-tetrafluoro-2-chloroethane (British Patent 1,062,768), or c) a mixture of chlorodifluoromethane and TFE formed by partial pyrolysis of chlorodifluoromethane to TFE, followed by removal of HCl (U.S. Pat. No. 3,459,818).        
U.S. Pat. No. 5,334,783 discloses a process for the preparation of hexafluoropropylene by thermal cleavage which uses a mixture (e.g., an azeotropic mixture) of chlorotetrafluoroethane and perfluorocyclobutane. Various reactor wall materials are mentioned including “platinum or similar noble metals”. The use of a platinum-tube reactor is exemplified. This patent suggests that where mixtures of chlorotetrafluoroethane isomers are employed, the ratio of CHClFCF3 to CHF2CClF2 is preferably not more than 1:4.
The prior art describing materials of construction for pyrolysis reactors used for producing TFE and HFP is non-specific, except that the material of construction has to withstand the reaction conditions and the chemical action of the reactants and reaction products. U.S. Pat. No. 3,306,940 discloses noble metals, silver, carbon, and Inconel® alloy, which is a nickel alloy. U.S. Pat. No. 2,551,573 compares performance in a carbon tube with and without a coil of gold wire in the reaction zone: the conversion rate and the product composition is the same in both cases. Inconel® alloy, because it is lower in cost than such metals as platinum and silver and lends itself to periodic cleaning to remove coke and/or polymer deposits, has become a standard material of construction for the pyrolysis reactor.
All of these processes can suffer from one or more of the problems of salts, of coke and/or polymer forming in the tubular pyrolysis reactor, eventually plugging the reactor and causing it to be shut down for cleaning, and of excessive by-product formation. These byproducts include perfluoroisobutylene (PFIB) (U.S. Pat. No. 5,705,719), which is toxic, and CF2═CFCl(CFC-1113), which is also undesirable in the process. Other products observed in some of these processes include the cyclic dimer of tetrafluoroethylene, namely perfluorocyclobutane, and 2-chloro-1,1,2,2-tetrafluoroethane. The sequential processes, i.e. first make TFE and then convert the TFE to HFP, while tending to provide high yields of these desirable perfluoroolefins, have the disadvantage of the extra expense of sequential operation and high production of the toxic PFIB and by-products in the second stage, plugging of the pyrolysis pipes/tubes and their high rate of erosion/corrosion causing low onstream efficiency and costly maintenance and shutdowns.
It is desirable to develop a reactor and a process for pyrolyzing fluoromonomer reactants for fluoropolymer production by which increased levels of such fluoromonomers are produced, while also reducing levels of undesirable by-products such as coke, polymer and PFIB.